1. Field of the Invention
The present invention relates to a pneumatic tire for heavy duty use. More particularly, the present invention pertains to a pneumatic tire for heavy duty use which is particularly well suited for constructional vehicles and the like and in which a carbon black-reinforced rubber composition is employed in a tread of the tire, said rubber composition being excellent in abrasion resistance and heat buildup performance (property of exhibiting low heat-buildup).
2. Description of the Related Arts
In order to improve the abrasion resistance of tread rubber of pneumatic tires, such improvements have heretofore been made by calling attention to the colloidal characteristics of reinforcing carbon black such as its particle diameter and structure (aggregates) For the improvement in carbon black for pneumatic tires, there is adopted in general, a method in which the interaction between carbon black and a polymer is enhanced by using carbon black having small particle sizes and high structures, and thus the reinforcing property is improved by the enhanced interaction therebetween. There is also adopted a method in which the blending amount of carbon black is increased to improve the abrasion resistance of the tread ruber.
In general, however, fine graining of carbon black is liable to deteriorate the low heat-buildup property, that is, heat buildup performance of a rubber composition. Thus, fine-grained carbon black gives rise to a serious problem when used for a heavy duty pneumatic tire such as a tire for a truck and a bus that are in use under severe conditions in many cases. In addition, the use of excessively fine-grained carbon black in a rubber composition brings about such problems as an increase in the amount of the phase wherein carbon black and a polymer are bound to each other, namely so-called carbon gel, accompanied by an increase in the viscosity of an unvulcanized rubber composition, and the resultant marked deterioration of the processability thereof. The use of the said carbon black further causes such a problem that the carbon black is poorly dispersed in a rubber composition with the result that the abrasion resistance thereof is adversely decreased.
In addition to the foregoing, when carbon black as a reinforcer is added to rubber in an excessive amount, there are raised a problem that the resultant rubber composition is unreasonably hardened, thereby making it difficult to carry out industrial processing, and at the same time, a problem in the physical properties of the rubber composition including inferior dispersion of the carbon black in the rubber composition and a failure to contrive enhancement of the abrasion resistance.
On the one hand, the use of carbon black having a high structure causes such a problem as unreasonable increase in the elastic modulus of the rubber composition, accompanied with deterioration of its fatigue resistance.
It follows from what has been mentioned hereinbefore, that the conventional methods for enhancing the abrasion resistance of a rubber composition by means of carbon black exert evil influences upon its low heat-buildup property, and particularly in the case of a heavy duty pneumatic tire, there has been an inevitable limit to the effect on improvement in its abrasion resistance.
On the other hand, there has recently been proposed as well a method for decreasing the proportion of large-diameter components of the aggregate that are considered to be the factors of inhibiting rubber enhancement, by sharpening the aggregate distribution of the primary particles of carbon black (refer to Japanese Patent Application Laid-Open Nos. 264647/1988 (Sho-63), 279624/1994 (Hei-6), etc.